PROCESS AND APPARATUS FOR PRODUCING LAMINATED MATERIALS

Abstract: A process and apparatus for producing laminated materials comprising a metal sheet laminated on one or both sides to a polymer film or films. The process comprises forming a laminate by a combination of heat and pressure followed by reheating. A heat zone is provided for heating and/or cooling the laminate after the reheat stage and prior to quenching so that the laminate enters the quenching stage at a substantially constant temperature, irrespective of line speed.

Description: PROCESS & APPARATUS FOR PRODUCING LAMINATED MATERIALS This invention relates to a process and apparatus for producing laminated materials comprising a metal sheet laminated on one or both sides to a polymer film or films. US-A-3679513 describes a process for applying polyethylene films to a steel substrate during a pre-determined heating cycle. The steel substrate is coated with chromium/chromium oxide and preheated so as to soften the polyethylene films which are applied to either side of the substrate between nip rolls. The resultant laminate is then reheated and temperature is maintained for from 5 to 240 seconds. Rapid cooling then takes place firstly by forced air to lower the temperature of the laminate to below the softening point of the polyethylene films, and finally by a water spray. In situations where fast line speeds are not of commercial importance, US-A-3679513 suggests cooling in still air. Both the rapid and the still air cooling methods of US-A-3679513 lead to the polymeric film becoming at least partially crystalline. In particular, it has been found that forced air cooling below approximately 180°C or spray quenching from a higher temperature leads to "spotting" of coating. These cooling methods cool only where the air or spray strikes the coating so that only local cooling occurs and a mottled, partially crystalline coating results. Crystallinity is particularly to be avoided when laminated materials are to be shaped into articles such as cans or can ends. In the case of a polypropylene film, "crystallinity" means σ-form (rather than smectic) crystallinity. EP-B-.0312309 seeks to overcome the problems of US-A-3679513 and describes a process for laminating polypropylene film to a metal substrate. The resultant laminate is reheated to a temperature above the melting point of polypropylene and then uniformly and rapidly quenched by flooding the laminate with continuously cooled water which travels on the laminate to a collecting tank. Such rapid and uniform cooling seeks to prevent the polypropylene from having an o-form crystallinity and indeed provides a substantially smectic form of polypropylene. This form is less susceptible to deformations when shaped. A problem which arises on start-up of lamination processes when line speed is slow is that recrystallisation or, in the case of polypropylene, the formation of an x-form crystallinity can take place as cooling occurs after leaving the reheat zone prior to quenching. This recrystallisation is also exhibited when cooled in still air as suggested in US-A-3679513. This invention therefore seeks to provide a process and apparatus in which partial crystallinity and development of discontinuities are avoided. According to the invention, there is provided a process for producing a polymer/metal laminate comprising feeding a metal strip and a strip of polymer film to a lamination nip, heating the metal strip to a first temperature Tτ_ above the initial adhesion point of the polymer film to cause intimate contact of the film with the metal strip, reheating the resultant laminate to a second temperature T2 to cause the film to interact with and bond to the metal strip; allowing bonding to develop between the film and the metal strip while heating and/or cooling the laminate across its width; and quenching the laminate rapidly and uniformly, in which the laminate enters the quenching stage at a third temperature T3. Preferably, bonding is allowed to develop for at least one second while heating and/or cooling the laminate across its width. The heating and/or cooling stage thus ensures that the laminate enters the quenching stage at substantially the same temperature T3, irrespective of the line speed. It was found that the temperature T3 of the laminated material immediately before quenching was critical to ensuring that the polymer film, ie. the coating, remained amorphous or, in the case of polypropylene, in substantially smectic form. Particular temperatures T3 are required according to type of polymer film and preferably this temperature has a tolerance of 15°C, irrespective of line speed. Fine control of temperature T3 is preferably achieved by heating the laminate at low line speeds and cooling the laminate at higher line speeds. Heating may be by arrays of infra-red, hot air or induction heaters and cooling may be by one or more fans which blow air through cooling means comprising nozzles, slits, or arrays of holes. Both heaters and cooling means preferably extend across at least the whole width of the laminate and may be provided on both sides of the laminate. Heaters and coolers on both sides of the laminate are preferred where polymer films are applied to both sides. Advantageously, heating and/or cooling is by a column of heaters and cooling means, alternating along the column. These alternating columns have been found to be particularly advantageous in ensuring precise temperature conditions T3 prior to quenching. The heaters are preferably positioned between 20mm and 100mm from the laminate but may be moved towards or away from the laminate ouside this range as desired. A particular preferred position of the heaters from the laminate is 50mm. The cooling nozzles etc are typically positioned closer to the laminate than the heaters, for example about 10-τιm closer than the heaters. In a particularly preferred embodiment,the process further comprises monitoring the temperature T3 and adjusting power supplied to the heaters and/or fans so as to maintain temperature T3 within a 30 degree acceptable range (ie. + 15°C), irrespective of line speed. In another embodiment, the process may also include recirculating warm air. Recirculating may also include further heating of air to be blown onto the laminate. Alternatively, cooling may comprise blowing cool, fresh air or warming fresh air and blowing this warmed air onto the laminate. Usually, an exhaust fan is provided which expels air which is not to be recycled. According to another aspect of the present invention, there is provided an apparatus for producing a laminated material comprising a polymer film bonded to a metal substrate, said apparatus comprising means for feeding a metal strip and a strip of polymer film to a lamination nip, means for heating the metal strip to a first temperature Tτ_ above the initial adhesion point of the polymer film and to cause intimate contact of the film with the metal strip, means for reheating the resultant laminate to a second temperature T2 to cause the film to interact with and bond to the metal strip; heaters and coolers disposed adjacent to the laminate; sensing means for monitoring temperature across the width of the laminate as the laminate leaves said apparatus; and means for quenching the laminate rapidly and uniformly whereby the laminate enters the quenching means at a third temperature T3. Preferably, the apparatus includes means for feeding the metal strip and two strips of polymer film to the lamination nip. The heaters may be infra-red heaters or hot air or induction heaters, for example, and the coolers may be one or more fans in conjunction with nozzles, slits, or arrays holes across the width of the laminate. The heaters and nozzles are usually disposed alternately in a column, preferably with the nozzles nearer to the laminate than the heaters. The sensing means preferably comprises at least one pyrometer, for monitoring the temperature T3 and adjusting power supplied to the heaters and one or more fans. There may be an array of pyrometers, or other sensors, across the width of the laminate. The third temperature T3 thus remains substantially constant irrespective of line speed. In another preferred embodiment, the apparatus may include a recirculation damper. By adjusting the damper position, fresh or recirculated air may be blown through the nozzles, or a combination of both fresh and recirculated air. A preferred embodiment of process and apparatus for producing laminated materials will now be described by way of example only, with reference to the drawings, in which:- Figure 1 is a diagrammatic sketch of an apparatus for laminating three strips of material; Figure 2 is a longitudinal cross section of a heat zone of the apparatus of figure 1; and Figure 3 is the cross section of figure 2 with arrows indicating the direction of air flow. In figure 1 it can be seen that the apparatus comprises a first roll 10 over which a metal strip 15 is passed and second and third rolls 20, 30, over each of which a polymeric film strip 25, 35 respectively, is passed. Pinch rolls 40, 45 bring the metal strip 15 and polymeric film strips 25, 35 together and quenching apparatus 50 immerses the resultant laminate 60 in a copious flood of cooling liquid in accordance with EP-B-0319309. A preheater 100 is located between roll 10 and pinch rolls 40, 45 and serves to preheat the metal strip 15 to a temperature Ti above the initial adhesion point of the polymeric films before laminating at the pinch rolls 40,45. A second heater 110 reheats the laminate 60 to a temperature T2 higher than the preheat temperature T]_. Heat/cool zone 120 is located between heater 110 and the quenching apparatus 50 and serves to heat and/or cool the laminate 60, according to the line speed and laminate thickness, to ensure that the laminate 60 is at temperature T3 immediately prior to quenching, irrespective of line speed. The length of zone 120 is such that the laminate 60 will take at least one second to pass through the zone at maximum line speed. The quenching apparatus 50 comprises a reservoir 70 for containing a cooling liquid 75, a pump 80 to draw liquid from the reservoir 70, a heat exchanger 85 to cool liquid delivered by the pump and a trough 90 which receives cooled liquid from the heat exchanger 85. The laminate 60 passes through trough 90 and is entirely flooded edge to edge with cooled liquid. Alternatively, a preferred form of quenching apparatus comprises a pair of horizontal distibutor bars between which the laminate passes, generally vertically, and cooling liquid impinges at a small angle to the laminate from both sides, thus flooding the laminate with a continuously renewed supply of cooled liquid. Heat/cool zone 120 is shown in more detail in figure 2 and comprises a zone 200 through which the laminate 60 passes prior to quenching, which consists of a column of nozzles or slits 210 and heaters 220 on each side of the laminate. The columns are offset from each other and each row alternates nozzle-heater- nozzle etc, with a nozzle at each column end, so that a nozzle on one column opposes a heater on the other column, except at the ends. A pyrometer 230 monitors the temperature T3 of the laminate as it leaves the zone 200. Power supplied to the heaters 220 and variable speed fans 240 is controlled by automatic adjustment of power supplied to each of these in response to the pyrometer output. The fans 240 are variable speed so that the amount of fresh air drawn into the zone through air inlets 250 can be adjusted. s In the embodiment of figure 2, dampers 260 are provided which enable air to be recirculated by fans 240. Recirculated air passes from each fan 240 past thermocouple 270, which adjusts both the damper position and preheat heater 280, and then past the preheat heater 280 to the nozzle 220. An exhaust 290 allows fumes and heat build-up to be removed from the zone. Generally, the maximum width of the laminate 60 will be one metre with both heaters and nozzles extending beyond the edge of the laminate. The heaters will usually extend by at least 60mm beyond each edge and the nozzles by at least 65mm. It will be appreciated these sizes may vary according to the laminate width, availability of specific heaters and nozzles and width of metal/polymers available. The distance of the laminate strip from the heaters is variable. Typically, this distance will be 50mm and the nozzles are usually about 10mm forward of the heaters. Variation in these distances is achieved by moving the heaters and/or nozzles on a frame towards or away from the strip. Limits on these distances are given both by the physical constraints of the zone assembly and by heat transfer requirements. A range of from 20mm to 100mm for the heater distance from the laminate would generally be acceptable. Flow of air through the heat zone is shown in figure 3. Fresh air is drawn in through air inlets 250 by circulation fans 240. Thermocouple 270 monitors the temperature of the air and dampers 260 and preheat heaters 280 are adjusted accordingly. The air then enters inner chamber 300 from which it passes through nozzles 210 onto the laminate 60 as it travels through the zone 120. The laminate 60 travels through the centre of an outer chamber 310 from which air can be removed to exhaust 290 by means of a variable speed exhaust fan (not shown). The air to be extracted can travel from either end of the columns of heaters and nozzles as shown by the double arrows in figure 3. If the damper 260 is moved to the position shown in figure 2, warm air will be recirculated by fan 240 and its temperature monitored by thermocouple 270. Thermocouple 270 can be used to switch preheater 280 and/or to adjust the setting of damper 260. The laminate travels from top to bottom as indicated in figures 2 and 3 and its temperature 3 is monitored by pyrometer 230 as it leaves the zone. The temperature T3 which is required will vary according to the polymer films used but will be maintained substantially constant at both start up speed and as the line speed is increased to maximum. The time taken for the laminate to pass through the heat/cool zone is generally at least 1 second at maximum line speed but a major requirement of the zone is that it can cope with a wide range of line speeds of 8m/min to 80m/min or more. 8m/min is a typical start-up speed and at such a slow line speed it is imperative that the laminate be heated to avoid recrystallisation of the coating(s) or the return of a polypropylene film from smetic to <x-form and to ensure that T3 is controlled to + 15°C of its required value. As the line speed increases following start-up procedure, the amount of heat input required will decrease until it becomes necessary for power input to the heaters 220 to be reduced and/or the damper 260 to be adjusted to allow fresh cool air to be drawn by fan into heat zone 120 to cool the laminate. The thickness of the laminate will also dictate the heat and/or cooling required to achieve desired temperature T3 within a tolerance of +/-15°C. It can thus be seen that a number of variables are involved in the heat/cool zone, particular settings being given in the examples which follow. Example 1 5 A 0.2mm gauge steel strip was preheated and 15um PET film applied in the nip rolls to one side of the strip and a 40um polypropylene film to the other side so that initial adhesion of the films to the strip occurred. The resultant laminate was reheated to a temperature T2 of 10 255°C, this being the maximum temperature reached by the laminate during the lamination process. The laminate then entered the heat/cool zone and the heaters and/or fans were adjusted so that the laminate first contacted the quenching water at a temperature T3 of 215°C. The length 15 of the heat/cool zone was such that the time taken to travel from the exit of the reheat to the quenching stage was 2 seconds at maximum line speed. At start-up speed of 8m/min, full heating of about 30 to 40kW was applied in the heat/cool zone, once the strip 0 and film were moving. The fans were switched on at minimum so as to avoid the effects of inertia when cooling became required as the line speed increased. The exhaust fan was also on minimum to avoid over-cooling of the laminate by exhaust air at low line speed. 5 As the line speed was increased, T3 was monitored and heating reduced automatically until little or no heating was required and the fan speed was then increased as necessary to provide cooling to ensure T3 remained constant. The exhaust fan speed was also increased to 0 expel exhaust gases more quickly. Fine control of the heaters and fans ensured that temperature T3 remained at about 215°C for the laminate of this example for line speeds of from 8m/min to 80m/min. Example 2 A 0.17mm gauge steel strip was preheated and 20um polypropylene film applied in the nip rolls to one side of the strip and a 40um polypropylene film to the other side so that initial adhesion of the films to the strip occurred. The resultant lamminate was reheated to a temperature T2 of 230°C. In the heat/cool zone, the heaters and/or fans were adjusted automatically so that the laminate first contacted the quenching water at a temperature T3 of 200°C. For this example, full heating of 20 to 30kW was applied at start-up speed of 8m/min and reduced as the speed increased. Heating and/or cooling was adjusted automatically as in example 1 so as to ensure that T3 remained at about 200°C for line speeds of from 8m/min to 80m/min. It will be appreciated that the invention has been described above by way of example only and that changes may be made without departing from the scope of the invention.

Claims: C L A I M S : - 1. A process for producing a polymer/metal laminate comprising feeding a metal strip and a strip of polymer film to a lamination nip, heating the metal strip to a first temperature Ti above the initial adhesion point of the polymer film to cause intimate contact of the film with the metal strip, reheating the resultant laminate to a second temperature T2 to cause the film to interact with and bond to the metal strip; allowing bonding to develop between the film and the metal strip while heating and/or cooling the laminate across its width; and quenching the laminate rapidly and uniformly, in which the laminate enters the quenching stage at a third temperature T3. 2. A process according to claim 1, in which bonding is allowed to develop for at least one second while heating and/or cooling the laminate across ,its width. 3. A process according to claim 1 or claim 2, further comprising heating and/or cooling both sides of the laminate. 4. A process according to any one of claims 1 to 3, in which heating is by arrays of infra-red, hot air or induction heaters and cooling is by one or more fans which blow air through cooling means comprising nozzles, slits or arrays of holes. 5. A process according to claim 4, further comprising adjusting the distance of the heaters and/or cooling means from the laminate. 6. A process according to claim 5, comprising adjusting the distance of the heaters to between 20mm and 100mm from the laminate. 7. A process according to claim 5 or claim 6, comprising positioning the cooling means closer to the laminate than the heaters. 8. A process according to any one of claim 1 to 7, further comprising monitoring the temperature 3 and adjusting power supplied to the heaters and/or fans so as to maintain temperature T3 within a 30 degree range (ie. + 15°C), irrespective of line speed. 9. An apparatus for producing a laminated material comprising a polymer film bonded to a metal substrate, said apparatus comprising means for feeding a metal strip and a strip of polymer film to a lamination nip, means for heating the metal strip to a first temperature T]_ above the initial adhesion point of the polymer film and to cause intimate contact of the film with the metal strip, means for reheating the resultant laminate to a second temperature T2 to cause the film to interact with and bond to the metal strip; heaters and coolers disposed adjacent to the laminate; sensing means for monitoring temperature across the width of the laminate as the laminate leaves said apparatus; and means for quenching the laminate rapidly and uniformly whereby the laminate enters the quenching means at a third temperature T3. 10. An apparatus according to claim 9, in which the sensing means comprises at least one pyrometer, for monitoring the temperature T3 and adjusting power supplied to the heaters and/or coolers. 11. An apparatus according to claim 9 or claim 10, further comprising a recirculation damper.

Inventors: ROWLAND MARK JEREMY, ROWLAND, MARK, JEREMY
Applicants: METAL BOX PLC, ROWLAND MARK JEREMY, CARNAUDMETALBOX PLC, ROWLAND, MARK, JEREMY
Application year: 1994
Main classification: B32B31
IPC codes: B29C35, B29C47, B29C65, B29L9, B32B15, B32B37